PDCD1 and neoplasm: Programmed cell death 1 receptor (PD-1, also known as PDCD1, CD279) is a checkpoint molecule present on T cells, B cells, and NK cells, which can interact with its ligands: PD-L1 (CD274; expressed on tumor cells, fibroblasts, antigen presenting cells, and others) or PD-L2 (PDCD1LG2; present mainly on hematopoietic cells).